A variety of services are provided via a network due to the widespread use of a broadband network. Among these services, a streaming distribution service represented by a sport relay broadcasting is available. Streaming is not playback upon completion of downloading files of audio and video data via the Internet or the like, but playback while downloading the files.
Unicast of streaming data (one-to-one communication form which designates only one message destination address) requires user count-dependent resources such as a distribution server load and network band. It is difficult to apply streaming unicast to a large-scale service. Multicast (communication form for distributing identical data to a plurality of destination addresses) is a technique for implementing one-to-many (plurality) communication. Multicast is suitable for simultaneously transmitting data to many reception terminals as in streaming distribution.
Multicast does not have any mechanism of arrival confirmation or retransmission from a reception terminal side due to its nature. It is difficult to guarantee confirmation and quality of a reception status at each reception terminal. In particular, when multicast distribution is applied to a service with accounting, the quality management and guarantee must be indispensable functions for each recipient. The absence of these functions bottlenecks commercial utilization of multicast distribution.
Several references disclose conventional multicast techniques (e.g., Japanese Patent Laid-Open Nos. 2003-348133 and 2003-333577).
FIG. 7 is a block diagram showing the configuration of a general conventional multicast distribution system. A multicast distribution system 70 comprises a distribution server 71, a backbone network 73 having a plurality of routers 72a to 72m, and a plurality of reception terminals 74a to 74n. The distribution server 71 is connected to the router 72a, and at least one reception terminal 74 is connected to each of the routers 72b, . . . , 72m. In the specific example illustrated in FIG. 7, the reception terminals 74a and 72b are connected to the router 72b, and the reception terminal 74n is connected to the router 72m. 
The general multicast distribution system 70 shown in FIG. 7 distributes a multicast packet 79 from the distribution server 71 to the reception terminals 74a and 74b via the routers 72a and 72b. The distribution system also distributes the multicast packet 79 to the reception terminal 74n via the routers 72a and 72m. 